1. Field of the Invention
The present invention relates to a motor control apparatus for controlling rotation of a motor such as a brushless motor.
2. Description of the Related Art
The market is seeing increasing supply of products such as wireless LANs and mobile telephones that use electromagnetic waves as well as a variety of home electric appliances such as TVs and microwave ovens. Radiation noise emitted from these electronic devices is pervasive. In addition, the increasing use of large-scale facilities in plants, factories and offices has led to an increase in power consumption and in the production of radiation noise, and there will be no lessening of radiation noise owing to automation and the spread of personal computers. Furthermore, aging equipment is fast becoming a source of unnecessary radiation noise.
If new equipment is introduced under such conditions, the electromagnetic noise it will be subjected to is incalculable. In order to deal with this, the nations of the EC have taken the initiative and clarified methods and standards of immunity specifications as by the EN standard (EN55024/1998) put in the form of a recommendation, and this has become a standard for evaluating the performance of equipment. Further, there is increasing debate at various facilities such as ITU-T and CISPR, which are facilities that establish international standards. In particular, the CISPR (Comite International Special des Perturbations Radioelectriques) deals with the immunity problem through CISPR240.
[reference Non-Patent Document 1]: “Noise Immunity in Information Communication Equipment”, published by Corona K.K.
[reference Non-Patent Document 2]: “Boundary Values and Measurement Methods Relating to Immunity Characteristics of CISPR24—Information Technology Devices”, published by Science Forum K.K.
Conventional devices are evaluated according to a method compliant with the above-mentioned immunity standard and are provided with noise shields made of well-known metal materials. In addition, circuits are provided with capacitors or choke coils to alleviate the effects of exogenous noise. Further, devices equipped with a conventional brushless motor unit or brushless motor for dealing with changes in the electromagnetic environment are also similarly affected by exogenous noise. As a result, the rotational speed of the motor becomes unstable with regard to the target rpm.
Adding on a noise shield or electronic circuit filter is an example of a technique for preventing malfunction due to noise inflected upon equipment from the outside. A problem with such a countermeasure, however, is an increase in number of parts and a rise in cost.